The Pacific warm blob from July 1 to July 31, 2015. (NASA Earth Observatory)

Last year, a massive toxic algae bloom swept the West Coast, resulting in record-breaking levels of a potentially deadly brain-damaging chemical and closing fisheries from California to British Columbia. Since then, scientists have been investigating the causes of the event — and now, a group of researchers think they’ve figured out how the event, which they term “unprecedented,” happened.

The algae bloom was facilitated by an unusual “blob” of warm water lurking in the Pacific Ocean at that time, the scientists say. And while the blob itself may have been largely brought on by weird but natural ocean and atmospheric variations, climate change may result in similar conditions cropping up in the future — and hence, more frequent toxic algal blooms. The work was published in Geophysical Research Letters.

Algae blooms aren’t uncommon off the West Coast. In fact, they tend to happen around the same time every year, said the new study’s lead author Ryan McCabe, a research scientist at the University of Washington’s Joint Institute for the Study of the Atmosphere and Ocean.

Each spring, there’s a shift in winds and other atmospheric and oceanic conditions that causes deep, cold, nutrient-rich water to rise up from the seafloor to the surface of the ocean in a process known as “upwelling.” Having both lots of nutrients and sunlight at the water’s surface creates perfect conditions for algae to start multiplying.

Normally, these algae blooms are composed of many different species of phytoplankton, and most of them are not harmful. The difference in 2015 is that a single toxic species — Pseudo-nitzschia australis, which produces a neurotoxin called domoic acid — dominated the bloom, an event that McCabe says was “absolutely abnormal.” He and his colleagues are arguing that the strange warm blob is what made Pseudo-nitzchia’s take-over possible.

The blob first showed up in the Northeast Pacific in the fall of 2013. Hundreds of miles wide and about 300 feet deep, the blob’s waters were nearly 3 degrees Celsius warmer than what’s typical for the region. Scientists believe it persisted through the end of 2015 before fading away, although more recent research suggests that a remnant of the blob may still be lurking below the surface of the ocean.

While the appearance of the blob is still not completely understood, scientists believe it may have marked the beginning of a shift in a natural oceanic climate swing known as the Pacific Decadal Oscillation. It’s somewhat like a much longer-term version of the El Niño and La Niña cycle, phasing between warm and cool extremes in different parts of the Pacific.

Regardless of the cause, the unusually warm blob was a perfect set-up for the toxic algae. In the winter, before the upwelling starts along the coast, nutrients tend to be scarce at the ocean’s surface. But Pseudo-nitzschia are particularly well-suited to dealing with nutrient-poor conditions, according to McCabe, and they also grow much faster in warm water. As a result, they essentially were able to out-compete other species before the upwelling even began, leaving them poised for a massive bloom.

“We had a couple of samples off the coast before the spring transition, and those samples already had toxins in them,” McCabe noted. “And that is highly unusual. But because those few samples had those toxins, we said, ‘Oh wow, we already had a toxic species out there simply waiting for a big pulse of nutrients to explode.’”

Indeed, sampling off the coast indicated that concentrations of both Pseudo-nitzschia and the neurotoxin it produces peaked in early May, and did not drop off until later in the summer, when additional upwelling finally dragged other, less harmful, species up from deeper parts of the ocean.

To further investigate their theory, the researchers went back and looked at records from previous warm periods off the West Coast, resulting from El Nino events or other temporary climate shifts. The literature suggests that increases in Pseudo-nitzschia do seem to track these kinds of warming events.

“It seems as though there is this relationship with warming, and what we saw in 2015 was an extreme case of that,” said Stephanie Moore, a visiting scientist at NOAA’s Northwest Fisheries Science Center and a project scientist with the University Corporation for Atmospheric Research, who was not involved with the new study.

While 2015’s warm anomaly may have been the result of natural climate variability, the researchers have warned that future climate change could result in similar conditions, setting up the West Coast for more frequent toxic blooms.

“We really are looking at this event here in 2015 as a window into the future of what we might expect for conditions to look like,” McCabe said.

That said, McCabe added that temperature isn’t the only important factor when it comes to the occurrence of algae blooms. And Moore pointed out that future climate change is expected to influence all kinds of other variables that may contribute to changes in plankton behavior.

For instance, we may see changes in wind patterns, which are instrumental in starting the upwelling process each spring. These patterns may be complemented, or perhaps balanced out to a certain extent, by increasing stratification of the oceans — that is, the formation of warm and cold layers of water at different depths which sometimes get stuck that way, making upwelling more difficult.

And changes in the types and concentrations of nutrients that go into the ocean in the first place may also change as coastal populations increase. McCabe noted that changes in nutrient runoff are likely to be one of the biggest factors besides temperature affecting the severity of future algae blooms.